This invention relates to a composition and process for preventing a phenomenon known as "High Temperature Oxidation" during conventional heat treatments, particularly so-called "solution" heat treatments, of certain aluminum alloys.
After casting, cold working, or any other process that can promote inhomogeneous segregation of certain constituents of aluminum alloys, solid objects made of these alloys are often "solution" heat treated at temperature(s), between about 475 and 545.degree. C., that are believed to cause rehomogenization of all of the constituents of the alloy in a solid solution. This process is usually performed in a molten salt bath or in an air furnace, with the latter more commonly used because it is less expensive. When the objects treated are made of aluminum alloys of the 2000, 6000, and 7000 series and are treated in an air furnace, they are susceptible to the development of surface blisters, a very undesirable condition commonly known as "high temperature oxidation". It is known that this phenomenon is aggravated by the presence of water vapor and/or sulfur in the gaseous atmosphere surrounding the objects being heat treated. It is further known that high temperature oxidation can sometimes be prevented by including an open container of fluoroborate salt(s) in the same air furnace in which the heat treating is occurring. However, sometimes this method is ineffective, and even when it is effective in preventing high temperature oxidation, it also promotes a normally undesirable staining or darkening of the objects being heat treated. Furthermore, the effects of fluoroborate salt(s) can not be quickly removed from the gaseous atmosphere in the furnace by any known practical method, so that subsequent objects being processed, which may not be susceptible to high temperature oxidation and may need a bright surface, can be damaged by this treatment. Finally, the amount of fluoroborate salt(s) to be used must be carefully controlled, and an effective amount must be established by trial-and-error for each individual furnace, an obviously undesirable situation.
A major object of this invention is to prevent high temperature oxidation of aluminum alloy objects while avoiding or at least mitigating the undesirable features of current processes as described above. Another alternative or concurrent object is to achieve these improvements at minimum economic cost. Other objects will be apparent from the description below.
Except in the claims and the operating examples, or where otherwise expressly indicated, all numerical quantities in this description indicating amounts of material or conditions of reaction and/or use are to be understood as modified by the word "about" in describing the broadest scope of the invention. Practice within the numerical limits stated is generally preferred, however. Also, throughout the specification, unless expressly stated to the contrary: percent, "parts of", and ratio values are by weight; the description of a group or class of materials as suitable or preferred for a given purpose in connection with the invention implies that mixtures of any two or more of the members of the group or class are equally suitable or preferred; description of constituents in chemical terms refers to the constituents at the time of addition to any combination specified in the description or of generation within the combination, from one or more other material(s) added to the combination by chemical reaction(s) noted in the description that are known or believed to occur between or among specific newly added constituent(s) and other constituent(s) already present in the combination, and does not necessarily preclude unspecified chemical interactions among the constituents of a mixture once mixed; specification of materials in ionic form implies the presence of sufficient counterions to produce electrical neutrality for the composition as a whole; any counterions thus implicitly specified should preferably be selected from among other constituents explicitly specified in ionic form, to the extent possible; otherwise such counterions may be freely selected, except for avoiding counterions that act adversely to the object(s) of the invention; the terms "molecule" and "mole" and their grammatical variations may be applied to ionic, elemental, or any other type of chemical entities defined by the number of atoms of each type present therein, as well as to substances with well-defined neutral molecules; the first definition of an acronym or other abbreviation applies to all subsequent uses herein of the same abbreviation and applies mutatis mutandis to normal grammatical variations of the initially defined abbreviation; the term "paint" includes all like materials that may be designated by more specialized terms such as lacquer, enamel, varnish, shellac, and the like; and the term "polymer" includes "oligomer", "homopolymer", "copolymer", "terpolymer", and the like.